1. Introduction
Nutrients may be one of the causative factors of oxidative stress. They cause redox imbalance and further lead to a number of diseases produced through accumulating reactive oxygen species (ROS)
in vivo [
1–
3]. Protein is one of three major nutrients in organisms. Excessive protein ingestion can increase amino acid oxidation and urea synthesis [
4] and decrease the nutritional efficiency of energy utilization [
5]. However, the reoxidation of reducing equivalents derived from amino acid oxidation is linked to the mitochondrial redox chain [
6]. Free radical generation during mitochondrial oxygen reduction may lead to oxidative stress if the antioxidant capacity is insufficient to quench the extra free radical production, and thereby endangers human health [
7,
8]. Gu
et al. [
9] found that a high-protein diet could destroy the balance of oxidation and antioxidants in the digestive system of mice and increase level of ROS in the digestive gland. The pancreas is an important glandular organ in the digestive system, and the normal operation of its function is essential for the digestion and absorption of nutrients. Moreover, many studies have confirmed that pathogeny of acute pancreatitis, which is one of the diseases with higher incidence, is attributed to the oxidative damage of free radicals on endocrine and exocrine function of pancreas [
10,
11]. In this regard, the present study was designed to investigate the oxidative damage of excessive protein ingestion on pancreas function in mice.
4. Discussion
It is reported that a high-protein diet could result in an imbalance between oxidation and antioxidants in the digestive system of mice and increase the level of ROS in pancreas [
9]. Oxidative stress is one of the causative factors of many diseases such as atherosclerosis [
24]. An imbalance between production of free radicals and antioxidant levels leads to oxidative stress, which is obvious from the depressed antioxidant defense system in the EPD group of our study. Cysteamine acts as an antioxidant due to its sulfhydryl in relation to effectively scavenging free radicals (e.g., Hydroxyl radical) [
25]. In the present study, administration of cysteamine to EPD-fed mice prevented the buildup of oxidative stress by restoring normal activities of the enzymatic antioxidants SOD and GSH-Px and normal level of the T-AOC in pancreas; the concentrations of these antioxidants were decreased in the EPD-fed mice. The diminished antioxidant defense system in EPD-fed mice leads to damage of the so-called lipid peroxidation system. We observed increased concentration of MDA, indices of lipid peroxidation, in pancreas of EPD-fed animals. Administration of cysteamine decreased significantly the lipid peroxidation. This suggests that cysteamine played an antioxidant role in oxidative stress induced by the EPD.
Hara and Shiota [
26] reported that pancreas weight, protein and RNA contents were increased with an increased level of protein intake. We also obtained similar results. Our data indicates that ingestion of excessive protein diet led to a significant increase of pancreas/body weight, and protein, DNA and RNA contents were increased by 21.69%, 20.37% and 72.14% in pancreas of the EPD-fed mice compared with the NPD-fed mice. This result indicates that an excessive protein diet could stimulate cell differentiation and growth, and promote synthesis of protein, DNA and RNA in pancreas. A possible reason for this result is that ingestion of an excessive protein diet needs a large number of digestive enzymes synthesized and secreted by the pancreas in order to ensure the normal digestion and absorption of protein in the organism. However, the synthesis of DNA and RNA needs a large amount of ATP to synthesize purine and pyrimidine and activate amino acids, and the ATP-generating process is accompanied by free radical production, so that oxygen free radicals are excessively produced, which is the possible reason for oxidative stress induced by the excessive protein diet.
It is reported that a high protein diet is detrimental to renal functions and insulin sensitivity [
27–
29], and related to prostate cancer and calcium oxalate nephrolithiasis [
30,
31]. Therefore, ingestion of excessive protein can cause adverse effects to the healthy population. The endoplasmic reticulum system of endocrine and exocrine glands in the pancreas is subjected to free radical attack, so the pancreas is particularly sensitive to peroxidation damage [
32], which is obvious from the decreased digestive enzyme activities and the increased hormones levels in the EPD group of our study.
Digestive enzymes secreted by exocrine gland directly affect digestion and absorption of the various nutrients, thus, they are often used as reliable indicators for measuring exocrine glandular function of the pancreas. Our data showed a downtrend of digestive enzyme activities in mice fed with the EPD, and significant decrease of enzyme activities with an increase of free radicals levels. Administration of cysteamine to EPD-fed mice significantly increased the activity of digestive enzymes. Therefore, we presume that through inducing excessive production of oxygen free radicals, excessive protein may attack the cell membrane and lead to membrane damage, mainly including lipid peroxidation, DNA damage and protein degradation, which cause functional damage of acinar cells and decline of secretory function and finally results in pancreatic exocrine dysfunction. In addition, the reason that digestive enzyme activity was lowered may be related to regulation of somatostatin (SS) upon excessive production of oxygen free radicals, because SS could indirectly inhibit the production of oxygen free radicals by weakening secretion function of pancreatic gland alveolus and reducing the need of ATP, in order to maintain the balance between oxidant and antioxidant.
Insulin and SS secreted by islet cell are related to some current diseases with high incidence, for example, diabetes. Insulin can reflect the reserve and secretion function of islet cell. SS is an inhibitory regulation hormone, so it could reduce oxygen free radicals produced due to energy consumption in the absorption process by inhibiting exocrine pancreatic function [
33]. In the present study, levels of insulin and somatostatin were markedly higher in mice fed with the EPD compared with those fed with the NPD, which indicates that protein levels in the diet has an effect on secretion function of islet cells [
34]. In addition, we also found that with an increase in oxygen free radical contents in the pancreas, hormone levels showed a trend to increase; thus, we presume that on one hand, excessive free radicals may lead to oxidative damage of islet cell, while on the other hand, increase of oxygen free radicals may induce secretion of SS, which is attributed to SS’regulatory effect.